Electroplating solutions containing gold in the form of a soluble sulfite complex have been known since about 1962, see, e.g., Smith, U.S. Pat. No. 3,0059,789. As originally formulated, commercial solutions based on a gold-sulfite complex were stable only at pH values above about 8.0, and in practice were usually operated at pH range of from 9 to 11. In 1969, Meyer et al., in Swiss Patent No. 506,828, reported that in the presence of organic polyamines, notably ethylenediamine, sulfite-based plating solutions for gold-copper alloys could be stabilized at pH values as low as 6.5. More recently, Kikuchi et al., in the 1990 Japanese Kokai Tokkyo Koho JP 02,232,378 (90,232,378), reported a solution containing sodium gold sulfite which was stabilized at pH 8 by the presence of sodium 3-nitrobenzene sulfonate.
Other gold-sulfite complex plating compositions include the following:
Smith et al., U.S. Pat. No. 3,057,789, which discloses a cyanide free electrolytic bath containing a potassium or a sodium gold sulfite complex and disodium ethylenediamine tetracetate.
Smith et al., U.S. Pat. No. 3,666,640, entitled "Gold Plating Bath and Process," which discloses an aqueous bath for electroplating gold, comprising; alkali gold sulfite; an alkali sulfite; an alkali sulfate; a soluble compound of cadmium, copper, nickel, and/or arsenic; and an organic acid chelating agent, wherein the bath contains sufficient acid or alkali to adjust its pH to between 8.5 to 13.
Stevens, U.S. Pat. No. 4,048,023, which discloses a slightly alkaline gold plating solution, free of cyanide and phosphates, containing a sodium gold sulfite complex and a palladosamine chloride complex.
Laude et al., U.S. Pat. No. 4,192,723, which provides an aqueous solution comprising monovalent gold and ammonium sulfite complex.
Wilkinson, U.S. Pat. No. 4,366,035, which described a cyanide-free bath for the electrodeposition of gold alloys comprising an aqueous alkaline mixture of a gold sulfite, a water soluble copper alloying salt or complex, a water soluble palladium alloying salt or complex and an alkali metal sulfite or ammonium sulfite.
Baker et al., U.S. Pat. No. 4,435,253, which provides gold sulfite electroplating solutions comprising an alkali metal or ammonium gold sulfite, a water soluble salt of thallium metal, and a non-hydroxy, non-amino carboxylic acid.
Shemyakina et al., U.S. Pat. No. 4,497,696, which relates to a gold plating electrolyte which comprises the interaction of the reagents; chloroauric acid, salts of alkali metals of ethylenediamine tetraacetic acid, and alkali metal sulfite or ammonium sulfite.
Nakazawa et al., U.S. Pat. No. 4,717,459, which is directed to an electrolytic gold plating solution including a soluble gold salt, a conductivity salt and, in addition, a mixture of a lead compound and a complexing agent.
It is often desirable to operate gold electroplating solutions at pH values lower than neutral; as, for example, in plating on circuitry defined using alkaline-developable photoresists. It is characteristic of sulfite gold plating solutions that for stable operation a slight excess of sulfite ion beyond that required for complexation of the gold should be present in the solution. Additionally, however, the nature of the gold sulfite complex is such that for every ion of gold added to the solution two ions of sulfite are added. When gold is plated out of the solution at alkaline pH the excess sulfite remains, and can become oxidized to sulfate at the anode. Thus the dissolved solids content, and hence the specific gravity of typical sulfite gold plating solutions increases as the solutions are replenished. For certain applications such as high speed plating this characteristic tends to limit the operating lifetime of the solution. Sulfur dioxide begins to be evolved from sulfite-containing solutions at pH values below about 6.5, forming bisulfite ion, which can itself further react to sulfur dioxide and water. If a sulfite gold plating solution could be operated under stable control at pH values below about 6.5, the controlled evolution of sulfur dioxide could be used to remove a portion of the excess sulfite in a manner analogous to that by which excess cyanide is volatilized from acid gold cyanide electroplating systems.
Finally, it is well known in the art that various alloying, brightening and surface-conditioning agents tend to operate best in selective ranges of pH. Extending the operable pH range of sulfite gold plating solutions should thus increase their adaptability for use with such additives.